1. Field of the Invention
The present invention relates to an improved distillation process and apparatus whereby maximum sidedraw liquid product recovery is realized utilizing minimum energy. More particularly, the distillation process is controlled in a manner whereby the feed residence time in the feed stream heater, the temperature of the feed stream exiting the heater and the overflash in the distillation tower are mimimized while maintaining the required bottom product specification as well as other process constraints and desired operating conditions.
2. Description of the Prior Art
Distillation processes and apparatus wherein feed streams containing components to be separated are heated and then subjected to distillation whereby a liquid bottom product, one or more sidedraw liquid products, an overhead liquid product and overhead non-condensing gases are produced have been commonly utilized in refining and other similar applications. In such distillation processes where a liquid bottom product of particular specification is required, the objective is to recover as much material as possible from the feed as liquid products other than the bottom product while minimizing the energy required for carrying out the process. For example, in crude oil refining operations, reduced crude oil is commonly subjected to vacuum distillation to separate the crude oil into a bottom asphalt product meeting a minimum viscosity specification, one or more sidedraw liquid products, an overhead liquid product and non-condensing gases. The objective in the operation of such a crude oil distillation process is to recover the maximum quantity of low boiling point components in the liquid products other than the bottom product while satisfying the required bottom product minimum viscosity specification. This requires that the distillation process achieve maximum feed component separation while minimizing coking and and cracking of the feed stream as a result of its being heated.
Distillation processes and apparatus have been developed heretofore wherein inert gas is combined with the feed stream; the combined inert gas-feed stream is heated; the heated stream is introduced into a distillation tower wherein the feed stream is subjected to distillation at a low total tower pressure; inert gas is introduced into the distillation tower to lower the partial pressures of the feed stream components therein; and a liquid bottom product of required specification, one or more sidedraw liquid products, an overhead liquid product and overhead non-condensing gases are withdrawn from the distillation tower. While such processes and apparatus have been operated successfully, the flow rates of the various inert gas streams, the heating of the combined inert gas-feed stream and other process conditions have not been carefully controlled. As a result, maximum recovery of liquid products other than bottom liquid product has not been realized and excess energy has been expended.
By the present invention a distillation process and apparatus are provided wherein, through improved control of process conditions, feed stream heating as well as overall energy consumption are minimized and the recovery of overhead and sidedraw liquid products are maximized while meeting the required liquid bottom product specification.